1 //===- GlobalsModRef.cpp - Simple Mod/Ref Analysis for Globals ------------===//
3 // The LLVM Compiler Infrastructure
5 // This file is distributed under the University of Illinois Open Source
6 // License. See LICENSE.TXT for details.
8 //===----------------------------------------------------------------------===//
10 // This simple pass provides alias and mod/ref information for global values
11 // that do not have their address taken, and keeps track of whether functions
12 // read or write memory (are "pure"). For this simple (but very common) case,
13 // we can provide pretty accurate and useful information.
15 //===----------------------------------------------------------------------===//
17 #include "llvm/Analysis/Passes.h"
18 #include "llvm/ADT/SCCIterator.h"
19 #include "llvm/ADT/Statistic.h"
20 #include "llvm/Analysis/AliasAnalysis.h"
21 #include "llvm/Analysis/CallGraph.h"
22 #include "llvm/Analysis/MemoryBuiltins.h"
23 #include "llvm/Analysis/ValueTracking.h"
24 #include "llvm/IR/Constants.h"
25 #include "llvm/IR/DerivedTypes.h"
26 #include "llvm/IR/InstIterator.h"
27 #include "llvm/IR/Instructions.h"
28 #include "llvm/IR/IntrinsicInst.h"
29 #include "llvm/IR/Module.h"
30 #include "llvm/Pass.h"
31 #include "llvm/Support/CommandLine.h"
35 #define DEBUG_TYPE "globalsmodref-aa"
37 STATISTIC(NumNonAddrTakenGlobalVars,
38 "Number of global vars without address taken");
39 STATISTIC(NumNonAddrTakenFunctions,"Number of functions without address taken");
40 STATISTIC(NumNoMemFunctions, "Number of functions that do not access memory");
41 STATISTIC(NumReadMemFunctions, "Number of functions that only read memory");
42 STATISTIC(NumIndirectGlobalVars, "Number of indirect global objects");
44 // An option to enable unsafe alias results from the GlobalsModRef analysis.
45 // When enabled, GlobalsModRef will provide no-alias results which in extremely
46 // rare cases may not be conservatively correct. In particular, in the face of
47 // transforms which cause assymetry between how effective GetUnderlyingObject
48 // is for two pointers, it may produce incorrect results.
50 // These unsafe results have been returned by GMR for many years without
51 // causing significant issues in the wild and so we provide a mechanism to
52 // re-enable them for users of LLVM that have a particular performance
53 // sensitivity and no known issues. The option also makes it easy to evaluate
54 // the performance impact of these results.
55 static cl::opt<bool> EnableUnsafeGlobalsModRefAliasResults(
56 "enable-unsafe-globalsmodref-alias-results", cl::init(false), cl::Hidden);
59 /// FunctionRecord - One instance of this structure is stored for every
60 /// function in the program. Later, the entries for these functions are
61 /// removed if the function is found to call an external function (in which
62 /// case we know nothing about it.
63 struct FunctionRecord {
64 /// GlobalInfo - Maintain mod/ref info for all of the globals without
65 /// addresses taken that are read or written (transitively) by this
67 std::map<const GlobalValue *, unsigned> GlobalInfo;
69 /// MayReadAnyGlobal - May read global variables, but it is not known which.
70 bool MayReadAnyGlobal;
72 unsigned getInfoForGlobal(const GlobalValue *GV) const {
73 unsigned Effect = MayReadAnyGlobal ? AliasAnalysis::Ref : 0;
74 std::map<const GlobalValue *, unsigned>::const_iterator I =
76 if (I != GlobalInfo.end())
81 /// FunctionEffect - Capture whether or not this function reads or writes to
82 /// ANY memory. If not, we can do a lot of aggressive analysis on it.
83 unsigned FunctionEffect;
85 FunctionRecord() : MayReadAnyGlobal(false), FunctionEffect(0) {}
88 /// GlobalsModRef - The actual analysis pass.
89 class GlobalsModRef : public ModulePass, public AliasAnalysis {
90 /// NonAddressTakenGlobals - The globals that do not have their addresses
92 std::set<const GlobalValue *> NonAddressTakenGlobals;
94 /// IndirectGlobals - The memory pointed to by this global is known to be
95 /// 'owned' by the global.
96 std::set<const GlobalValue *> IndirectGlobals;
98 /// AllocsForIndirectGlobals - If an instruction allocates memory for an
99 /// indirect global, this map indicates which one.
100 std::map<const Value *, const GlobalValue *> AllocsForIndirectGlobals;
102 /// FunctionInfo - For each function, keep track of what globals are
103 /// modified or read.
104 std::map<const Function *, FunctionRecord> FunctionInfo;
108 GlobalsModRef() : ModulePass(ID) {
109 initializeGlobalsModRefPass(*PassRegistry::getPassRegistry());
112 bool runOnModule(Module &M) override {
113 InitializeAliasAnalysis(this, &M.getDataLayout());
115 // Find non-addr taken globals.
119 AnalyzeCallGraph(getAnalysis<CallGraphWrapperPass>().getCallGraph(), M);
123 void getAnalysisUsage(AnalysisUsage &AU) const override {
124 AliasAnalysis::getAnalysisUsage(AU);
125 AU.addRequired<CallGraphWrapperPass>();
126 AU.setPreservesAll(); // Does not transform code
129 //------------------------------------------------
130 // Implement the AliasAnalysis API
132 AliasResult alias(const MemoryLocation &LocA,
133 const MemoryLocation &LocB) override;
134 ModRefResult getModRefInfo(ImmutableCallSite CS,
135 const MemoryLocation &Loc) override;
136 ModRefResult getModRefInfo(ImmutableCallSite CS1,
137 ImmutableCallSite CS2) override {
138 return AliasAnalysis::getModRefInfo(CS1, CS2);
141 /// getModRefBehavior - Return the behavior of the specified function if
142 /// called from the specified call site. The call site may be null in which
143 /// case the most generic behavior of this function should be returned.
144 ModRefBehavior getModRefBehavior(const Function *F) override {
145 ModRefBehavior Min = UnknownModRefBehavior;
147 if (FunctionRecord *FR = getFunctionInfo(F)) {
148 if (FR->FunctionEffect == 0)
149 Min = DoesNotAccessMemory;
150 else if ((FR->FunctionEffect & Mod) == 0)
151 Min = OnlyReadsMemory;
154 return ModRefBehavior(AliasAnalysis::getModRefBehavior(F) & Min);
157 /// getModRefBehavior - Return the behavior of the specified function if
158 /// called from the specified call site. The call site may be null in which
159 /// case the most generic behavior of this function should be returned.
160 ModRefBehavior getModRefBehavior(ImmutableCallSite CS) override {
161 ModRefBehavior Min = UnknownModRefBehavior;
163 if (const Function *F = CS.getCalledFunction())
164 if (FunctionRecord *FR = getFunctionInfo(F)) {
165 if (FR->FunctionEffect == 0)
166 Min = DoesNotAccessMemory;
167 else if ((FR->FunctionEffect & Mod) == 0)
168 Min = OnlyReadsMemory;
171 return ModRefBehavior(AliasAnalysis::getModRefBehavior(CS) & Min);
174 void deleteValue(Value *V) override;
175 void addEscapingUse(Use &U) override;
177 /// getAdjustedAnalysisPointer - This method is used when a pass implements
178 /// an analysis interface through multiple inheritance. If needed, it
179 /// should override this to adjust the this pointer as needed for the
180 /// specified pass info.
181 void *getAdjustedAnalysisPointer(AnalysisID PI) override {
182 if (PI == &AliasAnalysis::ID)
183 return (AliasAnalysis *)this;
188 /// getFunctionInfo - Return the function info for the function, or null if
189 /// we don't have anything useful to say about it.
190 FunctionRecord *getFunctionInfo(const Function *F) {
191 std::map<const Function *, FunctionRecord>::iterator I =
192 FunctionInfo.find(F);
193 if (I != FunctionInfo.end())
198 void AnalyzeGlobals(Module &M);
199 void AnalyzeCallGraph(CallGraph &CG, Module &M);
200 bool AnalyzeUsesOfPointer(Value *V, std::vector<Function *> &Readers,
201 std::vector<Function *> &Writers,
202 GlobalValue *OkayStoreDest = nullptr);
203 bool AnalyzeIndirectGlobalMemory(GlobalValue *GV);
207 char GlobalsModRef::ID = 0;
208 INITIALIZE_AG_PASS_BEGIN(GlobalsModRef, AliasAnalysis, "globalsmodref-aa",
209 "Simple mod/ref analysis for globals", false, true,
211 INITIALIZE_PASS_DEPENDENCY(CallGraphWrapperPass)
212 INITIALIZE_AG_PASS_END(GlobalsModRef, AliasAnalysis, "globalsmodref-aa",
213 "Simple mod/ref analysis for globals", false, true,
216 Pass *llvm::createGlobalsModRefPass() { return new GlobalsModRef(); }
218 /// AnalyzeGlobals - Scan through the users of all of the internal
219 /// GlobalValue's in the program. If none of them have their "address taken"
220 /// (really, their address passed to something nontrivial), record this fact,
221 /// and record the functions that they are used directly in.
222 void GlobalsModRef::AnalyzeGlobals(Module &M) {
223 std::vector<Function *> Readers, Writers;
224 for (Function &F : M)
225 if (F.hasLocalLinkage()) {
226 if (!AnalyzeUsesOfPointer(&F, Readers, Writers)) {
227 // Remember that we are tracking this global.
228 NonAddressTakenGlobals.insert(&F);
229 ++NumNonAddrTakenFunctions;
235 for (GlobalVariable &GV : M.globals())
236 if (GV.hasLocalLinkage()) {
237 if (!AnalyzeUsesOfPointer(&GV, Readers, Writers)) {
238 // Remember that we are tracking this global, and the mod/ref fns
239 NonAddressTakenGlobals.insert(&GV);
241 for (Function *Reader : Readers)
242 FunctionInfo[Reader].GlobalInfo[&GV] |= Ref;
244 if (!GV.isConstant()) // No need to keep track of writers to constants
245 for (Function *Writer : Writers)
246 FunctionInfo[Writer].GlobalInfo[&GV] |= Mod;
247 ++NumNonAddrTakenGlobalVars;
249 // If this global holds a pointer type, see if it is an indirect global.
250 if (GV.getType()->getElementType()->isPointerTy() &&
251 AnalyzeIndirectGlobalMemory(&GV))
252 ++NumIndirectGlobalVars;
259 /// AnalyzeUsesOfPointer - Look at all of the users of the specified pointer.
260 /// If this is used by anything complex (i.e., the address escapes), return
261 /// true. Also, while we are at it, keep track of those functions that read and
262 /// write to the value.
264 /// If OkayStoreDest is non-null, stores into this global are allowed.
265 bool GlobalsModRef::AnalyzeUsesOfPointer(Value *V,
266 std::vector<Function *> &Readers,
267 std::vector<Function *> &Writers,
268 GlobalValue *OkayStoreDest) {
269 if (!V->getType()->isPointerTy())
272 for (Use &U : V->uses()) {
273 User *I = U.getUser();
274 if (LoadInst *LI = dyn_cast<LoadInst>(I)) {
275 Readers.push_back(LI->getParent()->getParent());
276 } else if (StoreInst *SI = dyn_cast<StoreInst>(I)) {
277 if (V == SI->getOperand(1)) {
278 Writers.push_back(SI->getParent()->getParent());
279 } else if (SI->getOperand(1) != OkayStoreDest) {
280 return true; // Storing the pointer
282 } else if (Operator::getOpcode(I) == Instruction::GetElementPtr) {
283 if (AnalyzeUsesOfPointer(I, Readers, Writers))
285 } else if (Operator::getOpcode(I) == Instruction::BitCast) {
286 if (AnalyzeUsesOfPointer(I, Readers, Writers, OkayStoreDest))
288 } else if (auto CS = CallSite(I)) {
289 // Make sure that this is just the function being called, not that it is
290 // passing into the function.
291 if (!CS.isCallee(&U)) {
292 // Detect calls to free.
293 if (isFreeCall(I, TLI))
294 Writers.push_back(CS->getParent()->getParent());
296 return true; // Argument of an unknown call.
298 } else if (ICmpInst *ICI = dyn_cast<ICmpInst>(I)) {
299 if (!isa<ConstantPointerNull>(ICI->getOperand(1)))
300 return true; // Allow comparison against null.
309 /// AnalyzeIndirectGlobalMemory - We found an non-address-taken global variable
310 /// which holds a pointer type. See if the global always points to non-aliased
311 /// heap memory: that is, all initializers of the globals are allocations, and
312 /// those allocations have no use other than initialization of the global.
313 /// Further, all loads out of GV must directly use the memory, not store the
314 /// pointer somewhere. If this is true, we consider the memory pointed to by
315 /// GV to be owned by GV and can disambiguate other pointers from it.
316 bool GlobalsModRef::AnalyzeIndirectGlobalMemory(GlobalValue *GV) {
317 // Keep track of values related to the allocation of the memory, f.e. the
318 // value produced by the malloc call and any casts.
319 std::vector<Value *> AllocRelatedValues;
321 // Walk the user list of the global. If we find anything other than a direct
322 // load or store, bail out.
323 for (User *U : GV->users()) {
324 if (LoadInst *LI = dyn_cast<LoadInst>(U)) {
325 // The pointer loaded from the global can only be used in simple ways:
326 // we allow addressing of it and loading storing to it. We do *not* allow
327 // storing the loaded pointer somewhere else or passing to a function.
328 std::vector<Function *> ReadersWriters;
329 if (AnalyzeUsesOfPointer(LI, ReadersWriters, ReadersWriters))
330 return false; // Loaded pointer escapes.
331 // TODO: Could try some IP mod/ref of the loaded pointer.
332 } else if (StoreInst *SI = dyn_cast<StoreInst>(U)) {
333 // Storing the global itself.
334 if (SI->getOperand(0) == GV)
337 // If storing the null pointer, ignore it.
338 if (isa<ConstantPointerNull>(SI->getOperand(0)))
341 // Check the value being stored.
342 Value *Ptr = GetUnderlyingObject(SI->getOperand(0),
343 GV->getParent()->getDataLayout());
345 if (!isAllocLikeFn(Ptr, TLI))
346 return false; // Too hard to analyze.
348 // Analyze all uses of the allocation. If any of them are used in a
349 // non-simple way (e.g. stored to another global) bail out.
350 std::vector<Function *> ReadersWriters;
351 if (AnalyzeUsesOfPointer(Ptr, ReadersWriters, ReadersWriters, GV))
352 return false; // Loaded pointer escapes.
354 // Remember that this allocation is related to the indirect global.
355 AllocRelatedValues.push_back(Ptr);
357 // Something complex, bail out.
362 // Okay, this is an indirect global. Remember all of the allocations for
363 // this global in AllocsForIndirectGlobals.
364 while (!AllocRelatedValues.empty()) {
365 AllocsForIndirectGlobals[AllocRelatedValues.back()] = GV;
366 AllocRelatedValues.pop_back();
368 IndirectGlobals.insert(GV);
372 /// AnalyzeCallGraph - At this point, we know the functions where globals are
373 /// immediately stored to and read from. Propagate this information up the call
374 /// graph to all callers and compute the mod/ref info for all memory for each
376 void GlobalsModRef::AnalyzeCallGraph(CallGraph &CG, Module &M) {
377 // We do a bottom-up SCC traversal of the call graph. In other words, we
378 // visit all callees before callers (leaf-first).
379 for (scc_iterator<CallGraph *> I = scc_begin(&CG); !I.isAtEnd(); ++I) {
380 const std::vector<CallGraphNode *> &SCC = *I;
381 assert(!SCC.empty() && "SCC with no functions?");
383 if (!SCC[0]->getFunction()) {
384 // Calls externally - can't say anything useful. Remove any existing
385 // function records (may have been created when scanning globals).
386 for (auto *Node : SCC)
387 FunctionInfo.erase(Node->getFunction());
391 FunctionRecord &FR = FunctionInfo[SCC[0]->getFunction()];
393 bool KnowNothing = false;
394 unsigned FunctionEffect = 0;
396 // Collect the mod/ref properties due to called functions. We only compute
398 for (unsigned i = 0, e = SCC.size(); i != e && !KnowNothing; ++i) {
399 Function *F = SCC[i]->getFunction();
405 if (F->isDeclaration()) {
406 // Try to get mod/ref behaviour from function attributes.
407 if (F->doesNotAccessMemory()) {
408 // Can't do better than that!
409 } else if (F->onlyReadsMemory()) {
410 FunctionEffect |= Ref;
411 if (!F->isIntrinsic())
412 // This function might call back into the module and read a global -
413 // consider every global as possibly being read by this function.
414 FR.MayReadAnyGlobal = true;
416 FunctionEffect |= ModRef;
417 // Can't say anything useful unless it's an intrinsic - they don't
418 // read or write global variables of the kind considered here.
419 KnowNothing = !F->isIntrinsic();
424 for (CallGraphNode::iterator CI = SCC[i]->begin(), E = SCC[i]->end();
425 CI != E && !KnowNothing; ++CI)
426 if (Function *Callee = CI->second->getFunction()) {
427 if (FunctionRecord *CalleeFR = getFunctionInfo(Callee)) {
428 // Propagate function effect up.
429 FunctionEffect |= CalleeFR->FunctionEffect;
431 // Incorporate callee's effects on globals into our info.
432 for (const auto &G : CalleeFR->GlobalInfo)
433 FR.GlobalInfo[G.first] |= G.second;
434 FR.MayReadAnyGlobal |= CalleeFR->MayReadAnyGlobal;
436 // Can't say anything about it. However, if it is inside our SCC,
437 // then nothing needs to be done.
438 CallGraphNode *CalleeNode = CG[Callee];
439 if (std::find(SCC.begin(), SCC.end(), CalleeNode) == SCC.end())
447 // If we can't say anything useful about this SCC, remove all SCC functions
448 // from the FunctionInfo map.
450 for (auto *Node : SCC)
451 FunctionInfo.erase(Node->getFunction());
455 // Scan the function bodies for explicit loads or stores.
456 for (auto *Node : SCC) {
457 if (FunctionEffect == ModRef)
458 break; // The mod/ref lattice saturates here.
459 for (Instruction &I : inst_range(Node->getFunction())) {
460 if (FunctionEffect == ModRef)
461 break; // The mod/ref lattice saturates here.
463 // We handle calls specially because the graph-relevant aspects are
465 if (auto CS = CallSite(&I)) {
466 if (isAllocationFn(&I, TLI) || isFreeCall(&I, TLI)) {
467 // FIXME: It is completely unclear why this is necessary and not
468 // handled by the above graph code.
469 FunctionEffect |= ModRef;
470 } else if (Function *Callee = CS.getCalledFunction()) {
471 // The callgraph doesn't include intrinsic calls.
472 if (Callee->isIntrinsic()) {
473 ModRefBehavior Behaviour =
474 AliasAnalysis::getModRefBehavior(Callee);
475 FunctionEffect |= (Behaviour & ModRef);
481 // All non-call instructions we use the primary predicates for whether
482 // thay read or write memory.
483 if (I.mayReadFromMemory())
484 FunctionEffect |= Ref;
485 if (I.mayWriteToMemory())
486 FunctionEffect |= Mod;
490 if ((FunctionEffect & Mod) == 0)
491 ++NumReadMemFunctions;
492 if (FunctionEffect == 0)
494 FR.FunctionEffect = FunctionEffect;
496 // Finally, now that we know the full effect on this SCC, clone the
497 // information to each function in the SCC.
498 for (unsigned i = 1, e = SCC.size(); i != e; ++i)
499 FunctionInfo[SCC[i]->getFunction()] = FR;
503 /// alias - If one of the pointers is to a global that we are tracking, and the
504 /// other is some random pointer, we know there cannot be an alias, because the
505 /// address of the global isn't taken.
506 AliasResult GlobalsModRef::alias(const MemoryLocation &LocA,
507 const MemoryLocation &LocB) {
508 // Get the base object these pointers point to.
509 const Value *UV1 = GetUnderlyingObject(LocA.Ptr, *DL);
510 const Value *UV2 = GetUnderlyingObject(LocB.Ptr, *DL);
512 // If either of the underlying values is a global, they may be non-addr-taken
513 // globals, which we can answer queries about.
514 const GlobalValue *GV1 = dyn_cast<GlobalValue>(UV1);
515 const GlobalValue *GV2 = dyn_cast<GlobalValue>(UV2);
517 // If the global's address is taken, pretend we don't know it's a pointer to
519 if (GV1 && !NonAddressTakenGlobals.count(GV1))
521 if (GV2 && !NonAddressTakenGlobals.count(GV2))
524 // If the two pointers are derived from two different non-addr-taken
525 // globals we know these can't alias.
526 if (GV1 && GV2 && GV1 != GV2)
529 // If one is and the other isn't, it isn't strictly safe but we can fake
530 // this result if necessary for performance. This does not appear to be
531 // a common problem in practice.
532 if (EnableUnsafeGlobalsModRefAliasResults)
533 if ((GV1 || GV2) && GV1 != GV2)
536 // Otherwise if they are both derived from the same addr-taken global, we
537 // can't know the two accesses don't overlap.
540 // These pointers may be based on the memory owned by an indirect global. If
541 // so, we may be able to handle this. First check to see if the base pointer
542 // is a direct load from an indirect global.
544 if (const LoadInst *LI = dyn_cast<LoadInst>(UV1))
545 if (GlobalVariable *GV = dyn_cast<GlobalVariable>(LI->getOperand(0)))
546 if (IndirectGlobals.count(GV))
548 if (const LoadInst *LI = dyn_cast<LoadInst>(UV2))
549 if (const GlobalVariable *GV = dyn_cast<GlobalVariable>(LI->getOperand(0)))
550 if (IndirectGlobals.count(GV))
553 // These pointers may also be from an allocation for the indirect global. If
554 // so, also handle them.
555 if (AllocsForIndirectGlobals.count(UV1))
556 GV1 = AllocsForIndirectGlobals[UV1];
557 if (AllocsForIndirectGlobals.count(UV2))
558 GV2 = AllocsForIndirectGlobals[UV2];
560 // Now that we know whether the two pointers are related to indirect globals,
561 // use this to disambiguate the pointers. If the pointers are based on
562 // different indirect globals they cannot alias.
563 if (GV1 && GV2 && GV1 != GV2)
566 // If one is based on an indirect global and the other isn't, it isn't
567 // strictly safe but we can fake this result if necessary for performance.
568 // This does not appear to be a common problem in practice.
569 if (EnableUnsafeGlobalsModRefAliasResults)
570 if ((GV1 || GV2) && GV1 != GV2)
573 return AliasAnalysis::alias(LocA, LocB);
576 AliasAnalysis::ModRefResult
577 GlobalsModRef::getModRefInfo(ImmutableCallSite CS, const MemoryLocation &Loc) {
578 unsigned Known = ModRef;
580 // If we are asking for mod/ref info of a direct call with a pointer to a
581 // global we are tracking, return information if we have it.
582 const DataLayout &DL = CS.getCaller()->getParent()->getDataLayout();
583 if (const GlobalValue *GV =
584 dyn_cast<GlobalValue>(GetUnderlyingObject(Loc.Ptr, DL)))
585 if (GV->hasLocalLinkage())
586 if (const Function *F = CS.getCalledFunction())
587 if (NonAddressTakenGlobals.count(GV))
588 if (const FunctionRecord *FR = getFunctionInfo(F))
589 Known = FR->getInfoForGlobal(GV);
591 if (Known == NoModRef)
592 return NoModRef; // No need to query other mod/ref analyses
593 return ModRefResult(Known & AliasAnalysis::getModRefInfo(CS, Loc));
596 //===----------------------------------------------------------------------===//
597 // Methods to update the analysis as a result of the client transformation.
599 void GlobalsModRef::deleteValue(Value *V) {
600 if (GlobalValue *GV = dyn_cast<GlobalValue>(V)) {
601 if (NonAddressTakenGlobals.erase(GV)) {
602 // This global might be an indirect global. If so, remove it and remove
603 // any AllocRelatedValues for it.
604 if (IndirectGlobals.erase(GV)) {
605 // Remove any entries in AllocsForIndirectGlobals for this global.
606 for (std::map<const Value *, const GlobalValue *>::iterator
607 I = AllocsForIndirectGlobals.begin(),
608 E = AllocsForIndirectGlobals.end();
610 if (I->second == GV) {
611 AllocsForIndirectGlobals.erase(I++);
620 // Otherwise, if this is an allocation related to an indirect global, remove
622 AllocsForIndirectGlobals.erase(V);
624 AliasAnalysis::deleteValue(V);
627 void GlobalsModRef::addEscapingUse(Use &U) {
628 // For the purposes of this analysis, it is conservatively correct to treat
629 // a newly escaping value equivalently to a deleted one. We could perhaps
630 // be more precise by processing the new use and attempting to update our
631 // saved analysis results to accommodate it.
634 AliasAnalysis::addEscapingUse(U);